Exhaust purification system of internal combustion engine

ABSTRACT

In an internal combustion engine, inside of an engine exhaust passage, a hydrocarbon feed valve ( 15 ) and an exhaust purification catalyst ( 13 ) are arranged. The exhaust purification catalyst ( 13 ) is comprised of an upstream-side catalyst ( 14   a ) and a downstream-side catalyst ( 14   b ) arranged in series at an interval from each other. The upstream-side catalyst ( 14   b ) has a smaller cross-sectional area than the downstream-side catalyst ( 14   b ). The concentration of hydrocarbons which flow into the upstream-side catalyst ( 14   a ) is made to vibrate by within a predetermined range of amplitude of a 200 ppm or more and within a predetermined range of period of 5 seconds or less, whereby the NO x  which is contained in exhaust gas is reduced at the exhaust purification catalyst ( 13 ).

TECHNICAL FIELD

The present invention relates to an exhaust purification system of an internal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine which arranges, in an engine exhaust passage, an NO_(x) storage catalyst which stores NO_(x) which is contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NO_(x) when the air-fuel ratio of the inflowing exhaust gas becomes rich, which arranges, in the engine exhaust passage upstream of the NO_(x) storage catalyst, an oxidation catalyst which has an adsorption function, and which feeds hydrocarbons into the engine exhaust passage upstream of the oxidation catalyst to make the air-fuel ratio of the exhaust gas flowing into the NO_(x) storage catalyst rich when releasing NO_(x) from the NO_(x) storage catalyst (for example, see Patent Literature 1).

In this internal combustion engine, the hydrocarbons which are fed when releasing NO_(x) from the NO_(x) storage catalyst are made gaseous hydrocarbons at the oxidation catalyst, and the gaseous hydrocarbons are fed to the NO_(x) storage catalyst. As a result, the NO_(x) which is released from the NO_(x) storage catalyst is reduced well.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent No. 3969450

SUMMARY OF INVENTION Technical Problem

However, there is the problem that when the NO_(x) storage catalyst becomes a high temperature, the NO_(x) purification rate falls.

An object of the present invention is to provide an exhaust purification system of an internal combustion engine which can obtain a high NO_(x) purification rate even if the temperature of the exhaust purification catalyst becomes a high temperature.

Solution to Problem

According to the present invention, there is provided an exhaust purification system of an internal combustion engine in which a hydrocarbon feed valve for feeding hydrocarbons is arranged inside of an engine exhaust passage, an exhaust purification catalyst for reacting NO_(x) contained in exhaust gas and reformed hydrocarbons is arranged inside of the engine exhaust passage downstream of the hydrocarbon feed valve, the exhaust purification catalyst is comprised of an upstream-side catalyst and a downstream-side catalyst arranged in series at an interval from each other, the upstream-side catalyst has a smaller cross-sectional area than the downstream-side catalyst and has a function of at least reforming hydrocarbons which are fed from the hydrocarbon feed valve, a precious metal catalyst is carried on an exhaust flow surface of at least one catalyst of the upstream-side catalyst and the downstream-side catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalyst, the exhaust purification catalyst has a property of reducing the NO_(x) which is contained in exhaust gas if a concentration of hydrocarbons which flow to the upstream-side catalyst is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period and has a property of being increased in storage amount of NO_(x) which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than the predetermined range, and, at the time of engine operation, the concentration of hydrocarbons which flow to the upstream-side catalyst is made to vibrate by within the above predetermined range of amplitude and within the above predetermined range of period to thereby reduce NO_(x) which is contained in exhaust gas in the exhaust purification catalyst.

Advantageous Effects of Invention

Even if the temperature of the exhaust purification catalyst becomes a high temperature, a high NO_(x) purification rate can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an overall view of a compression ignition type internal combustion engine.

FIG. 2 is a view schematically showing a surface part of a catalyst carrier.

FIG. 3 is a view for explaining an oxidation reaction in an exhaust purification catalyst.

FIG. 4 is a view showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.

FIG. 5 is a view showing an NO_(x) purification rate.

FIGS. 6A, 6B, and 6C are views for explaining an oxidation reduction reaction in an exhaust purification catalyst.

FIGS. 7A and 7B are views for explaining an oxidation reduction reaction in an exhaust purification catalyst.

FIG. 8 is a view showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.

FIG. 9 is a view of an NO_(x) purification rate.

FIG. 10 is a time chart showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.

FIG. 11 is a time chart showing a change of an air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst.

FIG. 12 is a view showing a relationship between an oxidizing strength of an exhaust purification catalyst and a demanded minimum air-fuel ratio X.

FIG. 13 is a view showing a relationship between an oxygen concentration in exhaust gas and an amplitude ΔH of a hydrocarbon concentration giving the same NO_(x) purification rate.

FIG. 14 is a view showing a relationship between an amplitude ΔH of a hydrocarbon concentration and an NO_(x) purification rate.

FIG. 15 is a view showing a relationship of a vibration period ΔT of a hydrocarbon concentration and an NO_(x) purification rate.

FIG. 16 is a view showing a map of the hydrocarbon feed amount W.

FIG. 17 is a view showing a change in the air-fuel ratio of the exhaust gas flowing to the exhaust purification catalyst etc.

FIG. 18 is a view showing a map of an exhausted NO_(x) amount NOXA.

FIG. 19 is a view showing a fuel injection timing.

FIG. 20 is a view showing a map of a hydrocarbon feed amount WR.

FIGS. 21A and 21B are view showing enlarged views of an exhaust purification catalyst.

FIG. 22 is a flow chart for NO_(x) purification control.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is an overall view of a compression ignition type internal combustion engine.

Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electronically controlled fuel injector for injecting fuel into each combustion chamber 2, 4 an intake manifold, and 5 an exhaust manifold. The intake manifold 4 is connected through an intake duct 6 to an outlet of a compressor 7 a of an exhaust turbocharger 7, while an inlet of the compressor 7 a is connected through an intake air amount detector 8 to an air cleaner 9. Inside the intake duct 6, a throttle valve 10 driven by a step motor is arranged. Furthermore, around the intake duct 6, a cooling device 11 is arranged for cooling the intake air which flows through the inside of the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided to the inside of the cooling device 11 where the engine cooling water is used to cool the intake air.

On the other hand, the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7 b of the exhaust turbocharger 7. The outlet of the exhaust turbine 7 b is connected through an exhaust pipe 12 to an exhaust purification catalyst 13. As shown in FIG. 1, this exhaust purification catalyst 13 is comprised of an upstream-side catalyst 14 a and a downstream-side catalyst 14 b arranged in series at an interval from each other. The upstream-side catalyst 14 a has a smaller cross-sectional area than the downstream-side catalyst 14 b′.

Inside the exhaust pipe 12 upstream of the exhaust purification catalyst 13, a hydrocarbon feed valve 15 is arranged for feeding hydrocarbons comprised of diesel oil or other fuel used as fuel for a compression ignition type internal combustion engine. In the embodiment shown in FIG. 1, diesel oil is used as the hydrocarbons which are fed from the hydrocarbon feed valve 15. Note that, the present invention can also be applied to a spark ignition type internal combustion engine in which fuel is burned under a lean air-fuel ratio. In this case, from the hydrocarbon feed valve 15, hydrocarbons comprised of gasoline or other fuel used as fuel of a spark ignition type internal combustion engine are fed.

On the other hand, the exhaust manifold 5 and the intake manifold 4 are connected with each other through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage 16. Inside the EGR passage 16, an electronically controlled EGR control valve 17 is arranged. Further, around the EGR passage 16, a cooling device 18 is arranged for cooling EGR gas flowing through the inside of the EGR passage 16. In the embodiment shown in FIG. 1, the engine cooling water is guided to the inside of the cooling device 18 where the engine cooling water is used to cool the EGR gas. On the other hand, each fuel injector 3 is connected through a fuel feed tube 19 to a common rail 20. This common rail 20 is connected through an electronically controlled variable discharge fuel pump 21 to a fuel tank 22. The fuel which is stored inside of the fuel tank 22 is fed by the fuel pump 21 to the inside of the common rail 20. The fuel which is fed to the inside of the common rail 20 is fed through each fuel feed tube 19 to the fuel injector 3.

An electronic control unit 30 is comprised of a digital computer provided with a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35, and an output port 36, which are connected with each other by a bidirectional bus 31. Downstream of the upstream-side catalyst 14 a, a temperature sensor 23 is attached for estimating a temperature of the upstream-side catalyst 14 a and a temperature of an upstream end of the upstream-side catalyst 14 a. The output signals of this temperature sensor 23 and intake air amount detector 8 are input through respectively corresponding AD converters 37 to the input port 35. Further, an accelerator pedal 40 has a load sensor 41 connected to it which generates an output voltage proportional to the amount of depression L of the accelerator pedal 40. The output voltage of the load sensor 41 is input through a corresponding AD converter 37 to the input port 35. Furthermore, at the input port 35, a crank angle sensor 42 is connected which generates an output pulse every time a crankshaft rotates by, for example, 15°. On the other hand, the output port 36 is connected through corresponding drive circuits 38 to each fuel injector 3, step motor for driving the throttle valve 10, hydrocarbon feed valve 15, EGR control valve 17, and fuel pump 21.

In a first embodiment according to the present invention, the upstream-side catalyst 14 a and downstream-side catalyst 14 b are formed from the same catalyst. FIG. 2 schematically shows the surface part of the catalyst carrier carried on the substrate of the upstream-side catalyst 14 a and downstream-side catalyst 14 b. In these upstream-side catalyst 14 a and downstream-side catalyst 14 b, as shown in FIG. 2, for example, precious metal catalysts 51 and 52 are carried on a catalyst carrier 50 comprised of alumina. Furthermore, on this catalyst carrier 50, a basic layer 53 is formed which includes at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth metal, a lanthanoid or another such rare earth and silver Ag, copper Cu, iron Fe, iridium Ir, or another metal able to donate electrons to NO_(x). The exhaust gas flows along the top of the catalyst carrier 50, so the precious metal catalysts 51 and 52 can be said to be carried on the exhaust gas flow surface of the upstream-side catalyst 14 a and downstream-side catalysts 4 b. Further, the surface of the basic layer 53 exhibits basicity, so the surface of the basic layer 53 is called the basic exhaust gas flow surface part 54.

On the other hand, in FIG. 2, the precious metal catalyst 51 is comprised of platinum Pt, while the precious metal catalyst 52 is comprised of rhodium Rh. That is, the precious metal catalysts 51 and 52 which are carried on the catalyst carrier 50 are comprised of platinum Pt and rhodium Rh. Note that, on the catalyst carrier 50 of the upstream-side catalyst 14 a and downstream-side catalyst 14 b, in addition to platinum Pt and rhodium Rh, palladium Pd may be further carried or, instead of rhodium Rh, palladium Pd may be carried. That is, the precious metal catalysts 51 and 52 which are carried on the catalyst carrier 50 are comprised of platinum Pt and at least one of rhodium Rh and palladium Pd.

If hydrocarbons are injected from the hydrocarbon feed valve 15 into the exhaust gas, the hydrocarbons are reformed at the upstream-side catalyst 14 a. In the present invention, at this time, the reformed hydrocarbons are used to remove the NO_(x) at the downstream-side catalyst 14 b. FIG. 3 schematically shows the modification action performed at the upstream-side catalyst 14 a at this time. As shown in FIG. 3, the hydrocarbons HC which are injected from the hydrocarbon feed valve 15 become radical hydrocarbons HC with a small carbon number by the catalyst 51.

FIG. 4 shows the timing of feeding hydrocarbons from the hydrocarbon feed valve 15 and the changes in the air-fuel ratio (A/F)in of the exhaust gas flowing into the upstream-side catalyst 14 a. Note that, the changes in the air-fuel ratio (A/F)in depend on the change in concentration of the hydrocarbons in the exhaust gas which flows into the upstream-side catalyst 14 a, so it can be said that the change in the air-fuel ratio (A/F)in shown in FIG. 4 expresses the change in concentration of the hydrocarbons. However, if the hydrocarbon concentration becomes higher, the air-fuel ratio (A/F)in becomes smaller, so, in FIG. 4, the more to the rich side the air-fuel ratio (A/F)in becomes, the higher the hydrocarbon concentration.

FIG. 5 shows the NO_(x) purification rate by the exhaust purification catalyst 13 with respect to the catalyst temperatures TC of the upstream-side catalyst 14 a when periodically making the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a change so as to, as shown in FIG. 4, make the air-fuel ratio (A/F)in of the exhaust gas flowing to the upstream-side catalyst 14 a change. The inventors engaged in research relating to NO_(x) purification for a long time. In the process of research, they learned that if making the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a vibrate by within a predetermined range of amplitude and within a predetermined range of period, as shown in FIG. 5, an extremely high NO_(x) purification rate is obtained even in a 400° C. or higher high temperature region.

Furthermore, at this time, a large amount of reducing intermediate containing nitrogen and hydrocarbons continues to be held or adsorbed on the surface of the basic layer 53 of the upstream-side catalyst 14 a, that is, on the basic exhaust gas flow surface part 54 of the upstream-side catalyst 14 a. It is learned that this reducing intermediate plays a central role in obtaining a high NO_(x) purification rate. Next, this will be explained with reference to FIGS. 6A, 6B, and 6C. Note that, FIGS. 6A and 6B schematically show the surface part of the catalyst carrier 50 of the upstream-side catalyst 14 a, while FIG. 6C schematically shows the surface part of the catalyst carrier 50 of the downstream-side catalyst 14 b. These FIGS. 6A, 6B, and 6C show the reaction which is presumed to occur when the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period.

FIG. 6A shows when the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a is low, while FIG. 6B shows when hydrocarbons are fed from the hydrocarbon feed valve 15 and the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a becomes high.

Now, as will be understood from FIG. 4, the air-fuel ratio of the exhaust gas which flows into the upstream-side catalyst 14 a is maintained lean except for an instant, so the exhaust gas which flows into the upstream-side catalyst 14 a normally becomes a state of oxygen excess. Therefore, the NO which is contained in the exhaust gas, as shown in FIG. 6A, is oxidized on the platinum 51 and becomes NO₂. Next, this NO₂ is further oxidized and becomes NO₃. Further part of the NO₂ becomes NO₂ ⁻. In this case, the amount of production of NO₃ is far greater than the amount of production of NO₂ ⁻. Therefore, on the platinum Pt 51, a large amount of NO₃ and a small amount of NO₂ ⁻ are produced. These NO₃ and NO₂ ⁻ are strong in activity. Below, these NO₃ and NO₂ ⁻ will be referred to as the active NO_(x)*.

On the other hand, if hydrocarbons are fed from the hydrocarbon feed valve 15, as shown in FIG. 3, the hydrocarbons are reformed and become radicalized inside of the upstream-side catalyst 14 a. As a result, as shown in FIG. 6B, the hydrocarbon concentration around the active NO_(x)* becomes higher. In this regard, after the active NO_(x)* is produced, if the state of a high oxygen concentration around the active NO_(x)* continues for a predetermined time or more, the active NO_(x)* is oxidized and is absorbed in the basic layer 53 in the form of nitrate ions NO₃ ⁻. However, if the hydrocarbon concentration around the active NO_(x)* is made higher before this predetermined time passes, as shown in FIG. 6B, the active NO_(x)* reacts on the platinum 51 with the radical hydrocarbons HC whereby a reducing intermediate is produced on the surface of the basic layer 53.

Note that, at this time, the first produced reducing intermediate is considered to be a nitro compound R—NO₂. If this nitro compound R—NO₂ is produced, the result becomes a nitrile compound R—CN, but this nitrile compound R—CN can only survive for an instant in this state, so immediately becomes an isocyanate compound R—NCO. This isocyanate compound R—NCO, when hydrolyzed, becomes an amine compound R—NH₂. However, in this case, what is hydrolyzed is considered to be part of the isocyanate compound R—NCO. Therefore, as shown in FIG. 6B, the majority of the reducing intermediate which is produced on the surface of the basic layer 53 is believed to be the isocyanate compound R—NCO and amine compound R—NH₂. The reducing intermediate R—NCO or R—NH₂ which is produced at the upstream-side catalyst 14 a is sent to the downstream-side catalyst 14 b.

On the other hand, the cross-sectional area of the downstream-side catalyst 14 b is larger than the cross-sectional area of the upstream-side catalyst 14 a. Therefore, even if the air-fuel ratio of the exhaust gas which flows out from the upstream-side catalyst 14 a becomes rich for an instant, this rich gas will disperse before flowing into the downstream-side catalyst 14 b, therefore the air-fuel ratio of the exhaust gas which flows into the downstream-side catalyst 14 b is maintained constantly lean. Therefore, as shown in FIG. 6C, on the downstream-side catalyst 14 b, active NO_(x)* is actively produced. Further, part of the active NO_(x)* produced at the upstream-side catalyst 14 a, flows out from the upstream-side catalyst 14 a, flows into the downstream-side catalyst 14 b, and sticks or adheres to the surface of the basic layer 53 of the downstream-side catalyst 14 b. Therefore, a large amount of active NO_(x)* is held inside the downstream-side catalyst 14 b.

On the other hand, as explained before, a large amount of reducing intermediate is sent from the upstream-side catalyst 14 a to the downstream-side catalyst 14 b. The reducing intermediate R—NCO or R—NH₂, as shown in FIG. 6C, reacts with the active NO_(x)* which is held in the downstream-side catalyst 14 b and becomes N₂, CO₂, and H₂O, therefore the NO_(x) is removed.

In this way, in the exhaust purification catalyst 13, by making the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a temporarily higher and producing a reducing intermediate, the active NO₂* reacts with the reducing intermediate and the NO_(x) is removed. That is, in order for the exhaust purification catalyst 13 to remove the NO_(x), the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a has to be periodically changed.

Of course, in this case, it is necessary to raise the concentration of hydrocarbons to a concentration sufficiently high for producing the reducing intermediate. That is, it is necessary to make the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a vibrate by within a predetermined range of amplitude.

On the other hand, if lengthening the feed period of the hydrocarbons, the time in which the oxygen concentration becomes higher becomes longer in the period after the hydrocarbons are fed until the hydrocarbons are next fed. Therefore, the active NO_(x)* is absorbed in the basic layer 53 in the form of nitrates without producing a reducing intermediate. To avoid this, it is necessary to make the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 vibrate by within a predetermined range of period. Incidentally, in the example shown in FIG. 4, the injection interval is made 3 seconds.

As mentioned above, if making the vibration period of the hydrocarbon concentration, that is, the feed period of the hydrocarbons HC, longer than a predetermined range of period, the active NO_(x)* which is produced on the platinum Pt 53, as shown in FIG. 7A, will diffuse in the form of nitrate ions NO₃ ⁻ inside the basic layer 53 and become nitrates. That is, at this time, the NO_(x) in the exhaust gas is absorbed in the form of nitrates inside of the basic layer 53.

On the other hand, FIG. 7B shows the case when NO_(x) is absorbed in the form of nitrates inside the basic layer 53 in this way, the air-fuel ratio of the exhaust gas which flows into the upstream-side catalyst 14 a is made the stoichiometric air-fuel ratio or rich. In this case, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO₃ ⁻→NO₂), and consequently the nitrates absorbed in the basic layer 53 become nitrate ions NO₃ ⁻ one by one and, as shown in FIG. 7B, are released from the basic layer 53 in the form of NO₂. Next, the released NO₂ is reduced by the hydrocarbons HC and CO contained in the exhaust gas.

FIG. 8 shows the case of making the air-fuel ratio (A/F)in of the exhaust gas which flows into the upstream-side catalyst 14 a temporarily rich slightly before the NO_(x) absorption ability of the basic layer 53 becomes saturated. Note that, in the example shown in FIG. 8, the time interval of this rich control is 1 minute or more. In this case, the NO_(x) which was absorbed in the basic layer 53 when the air-fuel ratio (A/F)in of the exhaust gas was lean is released all at once from the basic layer 53 and reduced when the air-fuel ratio (A/F)in of the exhaust gas is made temporarily rich. Therefore, in this case, the basic layer 53 plays the role of an absorbent for temporarily absorbing NO_(x).

Note that, at this time, sometimes the basic layer 53 temporarily adsorbs the NO_(x). Therefore, if using term of storage as a term including both absorption and adsorption, at this time, the basic layer 53 performs the role of an NO_(x) storage agent for temporarily storing the NO_(x). That is, in this case, if referring to the ratio of air and fuel (hydrocarbons) which are fed into the engine intake passage, combustion chambers 2, and exhaust passage upstream of the upstream-side catalyst 14 a as the air-fuel ratio of the exhaust gas, the exhaust purification catalyst 13 functions as an NO_(x) storage catalyst which stores the NO_(x) when the air-fuel ratio of the exhaust gas is lean and releases the stored NO_(x) when the oxygen concentration in the exhaust gas falls.

FIG. 9 shows the NO_(x) purification rate when making the exhaust purification catalyst 13 function as an NO_(x) storage catalyst in this way. Note that, the abscissa of the FIG. 9 shows the catalyst temperature TC of the upstream-side catalyst 14 a. When making the exhaust purification catalyst 13 function as an NO_(x) storage catalyst, as shown in FIG. 9, when the catalyst temperature TC is 300° C. to 400° C., an extremely high NO_(x) purification rate is obtained, but when the catalyst temperature TC becomes a 400° C. or higher high temperature, the NO_(x) purification rate falls.

In this way, when the catalyst temperature TC becomes 400° C. or more, the NO_(x) purification rate falls because if the catalyst temperature TC becomes 400° C. or more, the nitrates break down by heat and are released in the form of NO₂ from the exhaust purification catalyst 13. That is, so long as storing NO_(x) in the form of nitrates, when the catalyst temperature TC is high, it is difficult to obtain a high NO_(x) purification rate. However, in the new NO_(x) purification method shown from FIG. 4 to FIGS. 6A and 6B, as will be understood from FIGS. 6A and 6B, nitrates are not formed or even if formed are extremely fine in amount, consequently, as shown in FIG. 5, even when the catalyst temperature TC is high, a high NO_(x) purification rate is obtained.

Therefore, in a first embodiment according to the present invention, a hydrocarbon feed valve 15 for feeding hydrocarbons is arranged inside of an engine exhaust passage, an exhaust purification catalyst 13 for reacting NO_(x) contained in exhaust gas and reformed hydrocarbons is arranged inside of the engine exhaust passage downstream of the hydrocarbon feed valve 15, the exhaust purification catalyst 13 is comprised of an upstream-side catalyst 14 a and downstream-side catalyst 14 b arranged in series at an interval from each other, the upstream-side catalyst 14 a has a smaller cross-sectional area than the downstream-side catalyst 14 b and has the function of reforming the hydrocarbons which are fed from the hydrocarbon feed valve 15, precious metal catalysts 51 and 52 are carried on the exhaust gas flow surface of the upstream-side catalyst 14 a and downstream-side catalyst 14 b and a basic exhaust gas flow surface part 54 is formed around the precious metal catalysts 51 and 52, the exhaust purification catalyst 13 has the property of reducing the NO_(x) which is contained in exhaust gas if the concentration of hydrocarbons which flow into the upstream-side catalyst 14 a is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in storage amount of NO_(x) which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than this predetermined range, and, at the time of engine operation, the concentration of hydrocarbons which flow into the upstream-side catalyst 14 a is made to vibrate by within the predetermined range of amplitude and within the predetermined range of period to thereby reduce the NO_(x) which is contained in the exhaust gas in the exhaust purification catalyst 13.

That is, the NO_(x) purification method which is shown from FIG. 4 to FIGS. 6A and 6B can be said to be a new NO_(x) purification method designed to remove NO_(x) without forming almost any nitrates in the case of using an exhaust purification catalyst which carries precious metal catalysts and forms a basic layer which can absorb NO_(x). In actuality, when using this new NO_(x) purification method, the nitrates which are detected from the basic layer 53 become much smaller in amount compared with the case where making the exhaust purification catalyst 13 function as an NO_(x) storage catalyst. Note that, this new NO_(x) purification method will be referred to below as the first NO_(x) purification method.

Next, referring to FIG. 10 to FIG. 15, this first NO_(x) purification method will be explained in a bit more detail.

FIG. 10 shows enlarged the change in the air-fuel ratio (A/F)in shown in FIG. 4. Note that, as explained before, the change of the air-fuel ratio (A/F)in of the exhaust gas which flows into the upstream-side catalyst 14 a simultaneously shows the change in the concentration of hydrocarbons which flow into the upstream-side catalyst 14 a. Note that, in FIG. 10, ΔH shows the amplitude of the change in concentration of hydrocarbons HC which flow into the upstream-side catalyst 14 a, while ΔT shows the vibration period of the concentration of the hydrocarbons which flow into the upstream-side catalyst 14 a.

Furthermore, in FIG. 10, (A/F)b shows the base air-fuel ratio which shows the air-fuel ratio of the combustion gas for generating the engine output. In other words, this base air-fuel ratio (A/F)b shows the air-fuel ratio of the exhaust gas which flows into the upstream-side catalyst 14 a when stopping the feed of hydrocarbons. On the other hand, in FIG. 10, X shows the upper limit of the air-fuel ratio (A/F)in used for producing the reducing intermediate without the produced active NO_(x)* being stored in the form of nitrates inside the basic layer 53 much at all, To make the active NO_(x)* and the reformed hydrocarbons react to produce a reducing intermediate, the air-fuel ratio (A/F)in has to be made lower than this upper limit X of the air-fuel ratio.

In other words, in FIG. 10, X shows the lower limit of the concentration of hydrocarbons required for making the active NO_(x)* and reformed hydrocarbon react to produce a reducing intermediate. To produce the reducing intermediate, the concentration of hydrocarbons has to be made higher than this lower limit X. In this case, whether the reducing intermediate is produced is determined by the ratio of the oxygen concentration and hydrocarbon concentration around the active NO_(x)*, that is, the air-fuel ratio (A/F)in. The upper limit X of the air-fuel ratio required for producing the reducing intermediate will below be called the demanded minimum air-fuel ratio.

In the example shown in FIG. 10, the demanded minimum air-fuel ratio X is rich, therefore, in this case, to form the reducing intermediate, the air-fuel ratio (A/F)in is instantaneously made the demanded minimum air-fuel ratio X or less, that is, rich. As opposed to this, in the example shown in FIG. 11, the demanded minimum air-fuel ratio X is lean. In this case, the air-fuel ratio (A/F)in is maintained lean while periodically reducing the air-fuel ratio (A/F)in so as to form the reducing intermediate.

In this case, whether the demanded minimum air-fuel ratio X becomes rich or becomes lean depends on the oxidizing strength of the upstream-side catalyst 14 a. In this case, the upstream-side catalyst 14 a, for example, becomes stronger in oxidizing strength if increasing the carried amount of the precious metal 51 and becomes stronger in oxidizing strength if strengthening the acidity. Therefore, the oxidizing strength of the upstream-side catalyst 14 a changes due to the carried amount of the precious metal 51 or the strength of the acidity.

Now, if using an upstream-side catalyst 14 a with a strong oxidizing strength, as shown in FIG. 11, if maintaining the air-fuel ratio (A/F)in lean while periodically lowering the air-fuel ratio (A/F)in, the hydrocarbons end up becoming completely oxidized when the air-fuel ratio (A/F)in is reduced. As a result, the reducing intermediate can no longer be produced. As opposed to this, when using an upstream-side catalyst 14 a with a strong oxidizing strength, as shown in FIG. 10, if making the air-fuel ratio (A/F)in periodically rich, when the air-fuel ratio (A/F)in is made rich, the hydrocarbons will be partially oxidized, without being completely oxidized, that is, the hydrocarbons will be reformed, consequently the reducing intermediate will be produced. Therefore, when using an upstream-side catalyst 14 a with a strong oxidizing strength, the demanded minimum air-fuel ratio X has to be made rich.

On the other hand, when using an upstream-side catalyst 14 a with a weak oxidizing strength, as shown in FIG. 11, if maintaining the air-fuel ratio (A/F)in lean while periodically lowering the air-fuel ratio (A/F)in, the hydrocarbons will be partially oxidized without being completely oxidized, that is, the hydrocarbons will be reformed and consequently the reducing intermediate will be produced. As opposed to this, when using an upstream-side catalyst 14 a with a weak oxidizing strength, as shown in FIG. 10, if making the air-fuel ratio (A/F)in periodically rich, a large amount of hydrocarbons will be exhausted from the upstream-side catalyst 14 a without being oxidized and consequently the amount of hydrocarbons which is wastefully consumed will increase. Therefore, when using an upstream-side catalyst 14 a with a weak oxidizing strength, the demanded minimum air-fuel ratio X has to be made lean.

That is, it is learned that the demanded minimum air-fuel ratio X, as shown in FIG. 12, has to be reduced the stronger the oxidizing strength of the upstream-side catalyst 14 a. In this way, the demanded minimum air-fuel ratio X becomes lean or rich due to the oxidizing strength of the upstream-side catalyst 14 a. Below, taking as example the case where the demanded minimum air-fuel ratio X is rich, the amplitude of the change in concentration of hydrocarbons flowing into the upstream-side catalyst 14 a and the vibration period of the concentration of hydrocarbons flowing into the upstream-side catalyst 14 a will be explained.

Now, if the base air-fuel ratio (A/F)b becomes larger, that is, if the oxygen concentration in the exhaust gas before the hydrocarbons are fed becomes higher, the feed amount of hydrocarbons required for making the air-fuel ratio (A/F)in the demanded minimum air-fuel ratio X or less increases. Therefore, the higher the oxygen concentration in the exhaust gas before the hydrocarbons are fed, the larger the amplitude of the hydrocarbon concentration has to be made.

FIG. 13 shows the relationship between the oxygen concentration in the exhaust gas before the hydrocarbons are fed and the amplitude ΔH of the hydrocarbon concentration when the same NO_(x) purification rate is obtained. From FIG. 13, it is learned that, to obtain the same NO_(x) purification rate, the higher the oxygen concentration in the exhaust gas before the hydrocarbons are fed, the greater the amplitude ΔH of the hydrocarbon concentration has to be made. That is, to obtain the same NO_(x) purification rate, the higher the base air-fuel ratio (A/F)b, the greater the amplitude ΔT of the hydrocarbon concentration has to be made. In other words, to remove the NO_(x) well, the lower the base air-fuel ratio (A/F)b, the more the amplitude ΔT of the hydrocarbon concentration can be reduced.

In this regard, the base air-fuel ratio (A/F)b becomes the lowest at the time of an acceleration operation. At this time, if the amplitude ΔH of the hydrocarbon concentration is about 200 ppm, it is possible to remove the NO_(x) well. The base air-fuel ratio (A/F)b is normally larger than the time of acceleration operation. Therefore, as shown in FIG. 14, if the amplitude ΔH of the hydrocarbon concentration is 200 ppm or more, an excellent NO_(x) purification rate can be obtained.

On the other hand, it is learned that when the base air-fuel ratio (A/F)b is the highest, if making the amplitude ΔH of the hydrocarbon concentration 10000 ppm or so, an excellent NO_(x) purification rate is obtained. Therefore, in the present invention, the predetermined range of the amplitude of the hydrocarbon concentration is made 200 ppm to 10000 ppm.

Further, if the vibration period ΔT of the hydrocarbon concentration becomes longer, the oxygen concentration around the active NO_(x)* becomes higher in the time after the hydrocarbons are fed to when the hydrocarbons are next fed. In this case, if the vibration period ΔT of the hydrocarbon concentration becomes longer than about 5 seconds, the majority of the active NO_(x)* starts to be absorbed in the form of nitrates inside the basic layer 53. Therefore, as shown in FIG. 15, if the vibration period ΔT of the hydrocarbon concentration becomes longer than about 5 seconds, the NO_(x) purification rate falls. Therefore, the vibration period ΔT of the hydrocarbon concentration has to be made 5 seconds or less.

On the other hand, if the vibration period ΔT of the hydrocarbon concentration becomes about 0.3 second or less, the fed hydrocarbons start to build up on the exhaust gas flow surface of the upstream-side catalyst 14 a, therefore, as shown in FIG. 15, if the vibration period ΔT of the hydrocarbon concentration becomes about 0.3 second or less, the NO_(x) purification rate falls. Therefore, in the present invention, the vibration period of the hydrocarbon concentration is made from 0.3 second to 5 seconds.

Now, in the present invention, by changing the hydrocarbon feed amount and injection timing from the hydrocarbon feed valve 15, the amplitude ΔH and vibration period ΔT of the hydrocarbons concentration is controlled so as to become the optimum values in accordance with the engine operating state. In this case, in this embodiment of the present invention, the hydrocarbon feed amount W able to give the optimum amplitude ΔH of the hydrocarbon concentration is stored as a function of the injection amount Q from the fuel injector 3 and engine speed N in the form of a map such as shown in FIG. 16 in advance in the ROM 32. Further, the optimum vibration amplitude ΔT of the hydrocarbon concentration, that is, the injection period ΔT of the hydrocarbons, is similarly stored as a function of the injection amount Q and engine speed N in the form of a map in advance in the ROM 32.

Next, referring to FIG. 17 to FIG. 20, an NO_(x) purification method in the case when making the exhaust purification catalyst 13 function as an NO_(x) storage catalyst will be explained in detail. The NO_(x) purification method in the case when making the exhaust purification catalyst 13 function as an NO_(x) storage catalyst in this way will be referred to below as the second NO_(x) purification method.

In this second NO_(x) purification method, as shown in FIG. 17, when the stored NO_(x) amount ΣNO_(x), which is stored in the basic layer 53 exceeds a predetermined allowable amount MAX, the air-fuel ratio (A/F)in of the exhaust gas which flows into the upstream-side catalyst 14 a is temporarily made rich. If the air-fuel ratio (A/F)in of the exhaust gas is made rich, the NO_(x) which was stored in the basic layer 53 when the air-fuel ratio (A/F)in of the exhaust gas was lean is released from the basic layer 53 all at once and reduced. Due to this, the NO_(x) is removed.

The stored NO_(x) amount ΣNO_(x), is, for example, calculated from the amount of NO_(x) which is exhausted from the engine. In this embodiment according to the present invention, the exhausted NO_(x) amount NOXA of NO_(x) which is exhausted from the engine per unit time is stored as a function of the injection amount Q and engine speed N in the form of a map such as shown in FIG. 18 in advance in the ROM 32. The stored NO_(x) amount ΣNO_(x), is calculated from exhausted NO_(x) amount NOXA. In this case, as explained before, the period in which the air-fuel ratio (A/F)in of the exhaust gas is made rich is usually 1 minute or more.

In this second NO_(x) purification method, as shown in FIG. 19, the fuel injector 3 injects additional fuel WR into the combustion chamber 2 in addition to the combustion-use fuel Q so that the air-fuel ratio (A/F)in of the exhaust gas flowing to the upstream-side catalyst 14 a is made rich. Note that, in FIG. 19, the abscissa indicates the crank angle. This additional fuel WR is injected at a timing at which it will burn, but will not appear as engine output, that is, slightly before ATDC90° after compression top dead center. This fuel amount WR is stored as a function of the injection amount Q and engine speed N in the form of a map such as shown in FIG. 20 in advance in the ROM 32. Of course, in this case, it is also possible to make the amount of feed of hydrocarbons from the hydrocarbon feed valve 15 increase so as to make the air-fuel ratio (A/F)in of the exhaust gas rich.

FIG. 21A shows an enlarged view of the surroundings of the exhaust purification catalyst 13 of FIG. 1 a. Further, FIG. 21B is a view for explaining the functions of the exhaust purification catalyst 13 according to the present invention shown in FIG. 21A.

Now, as explained before, to produce the reducing intermediate, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust purification catalyst 13 has to be made the demanded minimum air-fuel ratio X or less. In this case, as shown in FIG. 21B, if an enlarged cross-section part 55 of the exhaust passage is formed in front of the exhaust purification catalyst 13, the flow of the exhaust gas will be disturbed inside this enlarged cross-section part 55, so the hydrocarbons which are injected from the hydrocarbon feed valve 15 will end up diffusing in the radial direction and the flow direction. As a result, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 13 will end up greatly shifting to the lean side from the air-fuel ratio inside the exhaust pipe 12. Therefore, in this case, to make the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust purification catalyst 13 the demanded minimum air-fuel ratio X or less requires that a large amount of hydrocarbons be fed.

In the present invention, to reduce the amount of feed of hydrocarbons required for making the air-fuel ratio (A/F)in of the exhaust gas the demanded minimum air-fuel ratio X or less, as shown in FIG. 21A, the exhaust purification catalyst 13 is comprised of the upstream-side catalyst 14 a and downstream-side catalyst 14 b arranged in series at an interval from each other, and the upstream-side catalyst 14 a has a smaller cross-sectional area than the downstream-side catalyst 14 b. If making the cross-sectional area of the upstream-side catalyst 14 a smaller in this way, the degree of diffusion of the fed hydrocarbons in the exhaust gas which flows into the upstream-side catalyst 14 a becomes weaker, so it becomes possible to reduce the amount of feed of the hydrocarbons required for making the air-fuel ratio (A/F)in of the exhaust gas the demanded minimum air-fuel ratio X or less.

Note that, if making the diameter of the exhaust purification catalyst 13 as a whole smaller, it would be necessary to make the total length of the exhaust purification catalyst 13 greater. As a result, at the downstream side of the catalyst, the problem would arise of the catalyst temperature greatly falling. No only that, the problem would arise of the exhaust resistance greatly increasing. Therefore, in the present invention, to prevent these problems from arising, the diameter of the downstream-side catalyst 14 b is made larger and, as shown in FIG. 21A, an enlarged cross-section part 56 of the exhaust passage is formed between the upstream-side catalyst 14 a and downstream-side catalyst 14 b.

That is, in the present invention, the air-fuel ratio of the exhaust gas which flows to the downstream-side catalyst 14 b has not to be made the demanded minimum air-fuel ratio X or less. To produce NO_(x)*, that is, to raise the NO_(x) purification rate, it is necessary to maintain the air-fuel ratio of the exhaust gas which flows into the downstream-side catalyst 14 b lean. Therefore, as shown in FIG. 21A, it can be said to be preferable to form the enlarged cross-section part 56 between the upstream-side catalyst 14 a and downstream-side catalyst 14 b.

On the other hand, to prevent diffusion of the hydrocarbons which are injected from the hydrocarbon feed valve 15, it is necessary to prevent disturbance of the flow of exhaust gas flowing to the upstream-side catalyst 14 a as much as possible. Therefore, in this embodiment according to the present invention, as shown in FIG. 21A, the engine exhaust passage between the hydrocarbon feed valve 15 and the upstream-side catalyst 14 a is formed inside the straight extending exhaust pipe 12. In this case, to further prevent the fed hydrocarbons from diffusion, as shown in FIG. 21A, it is preferable to fit the upstream-side catalyst 14 a inside the exhaust pipe 12 of a constant diameter.

Note that, in the present invention, the upstream-side catalyst 14 a may be comprised of an oxidation catalyst, and just a partial oxidation action of hydrocarbons, that is, just a reforming action of the hydrocarbons may be performed in the upstream-side catalyst 14 a. In this case, production of the reducing intermediate and the purification action of the NO_(x) are performed in the downstream-side catalyst 14 b. Therefore, in the present invention, the upstream-side catalyst 14 a has a cross-sectional area smaller than the downstream-side catalyst 14 b and has the function of at least reforming the hydrocarbons which are fed from the hydrocarbon feed valve 15.

Further, in the present invention, as the downstream-side catalyst 14 b, for example, it is possible to use an NO_(x) purification catalyst in which a metal having a lower oxidizing strength than a precious metal is carried on a catalyst carrier. In this NO_(x) purification catalyst, for example, the catalyst carrier is comprised of alumina or zeolite, while the metal which is carried on this catalyst carrier is comprised of at least one transition metal selected from silver Ag, copper Cu, iron Fe, vanadium V, molybdenum Mo, cobalt Co, nickel Ni, and manganese Mn. Therefore, in the present invention, precious metal catalysts 51 and 52 are carried on the exhaust gas flow surface of at least one catalyst of the upstream-side catalyst 14 a and downstream-side catalyst 14 b, and a basic exhaust gas flow surface part 54 is formed around the precious metal catalysts 54 and 52.

Now, the oxidation reaction of hydrocarbons which flow into the upstream-side catalyst 14 a is performed most actively at the upstream end of the upstream-side catalyst 14 a. Therefore, at the upstream-side catalyst 14 a, the temperature of the upstream end becomes the highest. If the temperature of the upstream end of the upstream-side catalyst 14 a becomes high, the produced active NO_(x)* will start to be desorbed and as a result the amount of production of the reducing intermediate will start to fall, so the NO_(x) purification rate will start to fall. That is, the temperature TCA of the upstream end of the upstream-side catalyst 14 a has a predetermined limit temperature TC_(max) beyond which a drop in the NO_(x) purification rate is caused. This limit temperature TC_(max) is about 500° C.

Therefore, in this embodiment according to the present invention, when the temperature TCA of the upstream end of the upstream-side catalyst 14 a exceeds the predetermined limit temperature TC_(max) beyond which a drop in the NO_(x) purification rate is caused, the temperature TCA of the upstream end of the upstream-side catalyst 14 a is lowered. One method for lowering the temperature TCA of the upstream end of the upstream-side catalyst 14 a is the method of increasing the feed amount of hydrocarbons to make the atmosphere in the upstream-side catalyst 14 a rich. If making the atmosphere in the upstream-side catalyst 14 a rich, the oxidation reaction is suppressed and the heat of evaporation of the fed hydrocarbons causes the temperature TCA of the upstream end of the upstream-side catalyst 14 a to fall.

Further, other methods of lowering the temperature TCA of the upstream end of the upstream-side catalyst 14 a are to lengthen the vibration period ΔT of the concentration of hydrocarbons which flow into the upstream-side catalyst 14 a, that is, to lengthen the injection period of the hydrocarbons, or to stop the feed of hydrocarbons. In the present invention, either of these methods is used.

FIG. 22 shows the NO_(x) purification control routine. This routine is executed by interruption every constant time.

Referring to FIG. 22, first, at step 60, it is judged from the output signal of the temperature sensor 23 if the temperature TO of the upstream-side catalyst 14 a exceeds the activation temperature TX. When TC≧TX, that is, when the upstream-side catalyst 14 a is activated, the routine proceeds to step 61 where it is judged from the output signal of the temperature sensor 23 if the temperature TCA of the upstream end of the upstream-side catalyst 14 a exceeds the predetermined limit temperature TC_(max) beyond which a drop in the NO_(x) purification rate is caused. When TCA<TC_(max), it is judged that the first NO_(x) purification method should be used. At this time, the routine proceeds to step 62. At step 62, feed control of hydrocarbons from the hydrocarbon feed valve 15 is performed. At this time, the NO_(x) purification action by the first NO_(x) purification method is performed.

On the other hand, when it is judged at step 61 that TCA≧TC_(max), the routine proceeds to step 63 where a temperature drop processing in which the temperature TCA of the upstream end of the upstream-side catalyst 14 a is made to fall is performed. For example, the concentration of hydrocarbons which flow into the upstream-side catalyst 14 a is raised so that when the air-fuel ratio of the exhaust gas which flows into the upstream-side catalyst 14 a is lean, the air-fuel ratio of the exhaust gas is made to become rich, while when the air-fuel ratio of the exhaust gas which flows into the upstream-side catalyst 14 a is rich, the air-fuel ratio of the exhaust gas is made to become further rich. Alternatively, the vibration period of the concentration of hydrocarbons which flow into the upstream-side catalyst 14 a is made longer or the feed of hydrocarbons from the hydrocarbon feed valve 15 is stopped.

On the other hand, when it is judged at step 60 that TC<TX, it is judged that the second NO_(x) purification method should be used, then the routine proceeds to step 64. At step 64, the NO_(x) amount NOXA of the NO_(x) exhausted per unit time is calculated from the map shown in FIG. 18. Next, at step 65, ΣNO_(x) is increased by the exhausted NO_(x) amount NOXA to calculate the stored NO_(x) amount ΣNO_(x). Next, at step 66, it is judged if the stored NO_(x) amount ΣNO_(x) exceeds the allowable value MAX. When ΣNO_(x)>MAX, the routine proceeds to step 67 where the additional fuel amount WR is calculated from the map shown in FIG. 20 and an injection action of the additional fuel is performed. Next, at step 68, ΣNO_(x) is cleared.

REFERENCE SIGNS LIST

4 . . . intake manifold

5 . . . exhaust manifold

7 . . . exhaust turbocharger

12 . . . exhaust pipe

13 . . . exhaust purification catalyst

14 a . . . upstream-side catalyst

14 b . . . downstream-side catalyst

15 . . . hydrocarbon feed valve 

The invention claimed is:
 1. An exhaust purification system of an internal combustion engine, the exhaust purification system comprising: an engine exhaust passage; a hydrocarbon feed valve for feeding hydrocarbons arranged inside of the engine exhaust passage; an exhaust purification catalyst for reacting NO_(x) contained in exhaust gas and reformed hydrocarbons arranged inside of the engine exhaust passage downstream of the hydrocarbon feed valve, wherein the exhaust purification catalyst is comprised of an upstream-side catalyst and a downstream-side catalyst arranged in series and at an interval from each other, and the upstream-side catalyst has a smaller cross-sectional area than the downstream-side catalyst and has a function of at least reforming hydrocarbons that are fed from the hydrocarbon feed valve; a precious metal catalyst, wherein the precious metal catalyst is carried on an exhaust flow surface of at least one catalyst of the upstream-side catalyst, and the downstream-side catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalyst; and an electronic control unit, wherein the electronic control unit is configured to control a vibration of a concentration of hydrocarbons flowing into the exhaust purification catalyst within a predetermined range of amplitude and within a predetermined range of period, and is configured to control the vibration period of the hydrocarbon concentration longer than the predetermined range of period, wherein when the electronic control unit controls the vibration of the concentration of hydrocarbons flowing into the exhaust purification catalyst within the predetermined range of amplitude and within the predetermined range of period, the concentration of hydrocarbons flowing to the upstream-side catalyst vibrate within the predetermined range of amplitude and within the predetermined range of period, and the exhaust purification catalyst has a property of chemically reducing the NO_(x) that is contained in exhaust gas without storing, or storing a fine amount of nitrates in the exhaust purification catalyst, and when the electronic control unit controls the vibration period of the hydrocarbon concentration longer than the predetermined range period, the exhaust purification catalyst has a property of being increased in storage amount of NO_(x) that is contained in exhaust gas.
 2. The exhaust purification system of an internal combustion engine as claimed in claim 1, wherein the engine exhaust passage between the hydrocarbon feed valve and the upstream-side catalyst is formed inside a straight extending exhaust pipe.
 3. The exhaust purification system of an internal combustion engine as claimed in claim 1, wherein when a temperature of an upstream end of the upstream-side catalyst exceeds a predetermined limit temperature beyond which a drop in a NO_(x) purification rate is observed, the electronic control unit is further configured to lower the temperature of the upstream end of the upstream-side catalyst by raising the concentration of hydrocarbons that flow to the upstream-side catalyst so that an air-fuel ratio of the exhaust gas is made rich when the air-fuel ratio of the exhaust gas flowing into the upstream-side catalyst is lean, and the air-fuel ratio of the exhaust gas is made richer when the air-fuel ratio of the exhaust gas flowing into the upstream-side catalyst is rich.
 4. The exhaust purification system of an internal combustion engine as claimed in claim 1, wherein when a temperature of an upstream end of the upstream-side catalyst exceeds a predetermined limit temperature beyond which a drop in a NO_(x) purification rate is observed, the electronic control unit is further configured to lower the temperature of the upstream end of the upstream-side catalyst by lengthening the vibration period of the concentration of hydrocarbons that flow to the upstream-side catalyst or by stopping a feed of hydrocarbons from the hydrocarbon feed valve.
 5. The exhaust purification system of an internal combustion engine as claimed in claim 1, wherein NO_(x) contained in exhaust gas and reformed hydrocarbons are reacted inside the exhaust purification catalyst to produce a reducing intermediate containing nitrogen and hydrocarbons and wherein the vibration period of the hydrocarbon concentration is a vibration period required for continued production of the reducing intermediate.
 6. The exhaust purification system of an internal combustion engine as claimed in claim 5, wherein the vibration period of the hydrocarbon concentration is 0.3 second to 5 seconds.
 7. The exhaust purification system of an internal combustion engine as claimed in claim 1, wherein the precious metal catalyst is comprised of platinum (Pt) and at least one of rhodium (Rh) or palladium (Pd).
 8. The exhaust purification system of an internal combustion engine as claimed in claim 1, wherein a basic layer containing an alkali metal, an alkali earth metal, a rare earth, or a metal that can donate electrons to NO_(x) is formed on the exhaust gas flow surface of the exhaust purification catalyst, and wherein a surface of the basic layer forms the basic exhaust gas flow surface part. 